7423

• UCREFRP Catalog Number: 7423
• Author: U.S. Bureau of Reclamation.
• Title: Glen Canyon Environmental Studies Draft Report, September 1987.
• USFW Year: 1987.
• USFW - Doc Type: Washington, D.C.
• Copyright Material: NO

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and .reluctant to move off their spawning beds. Not
only are fish stranded during fluctuating flows, but
the spawning grounds are exposed, causing direct mor~
tality to eggs and young. Because as much as 28
percent of the trout harvest may depend on natural
reproduction, loss of eggs and young will reduce the
trout population. However, fluctuations increase the
availability of food to trout by increasing the
dislodgement and movement of invertebrates. The number
of invertebrates in trout stomachs increased sig-
nificantly during periods of fluctuating flow. (See
Appendix B, section v.)

Common native fish. Larval common native fishes are
relatively immobile, very susceptible to predation and
stranding, and require quiet, warm backwaters for
growth and survival. As flows fluctuate, the depth,
temperature, and velocity of backwaters change,
forcing fish to move into the mainstem river. This
increases the risk of predation and requires an ad-
ditional expenditure of energy (Appendix B, section IV
and V.)

Larval humpback chub are less dependent on backwaters
than common native fish because they do not move out of
the Little Colorado River until they are able to sur-
vive in the colder mainstem river. Fluctuating flows
probably have little effect on chub once they leave the
Little Colorado River, although some individuals may be
stranded when river levels change (Appendix B, section
V. )

Terrestrial resources. Terrestrial resources such as
beaches and vegetation are not strongly affected by
fluctuating flows. Because vegetation has stablized
above the level of flow fluctuation, changes in flow
within powerplant capacity have little effect on
terrestrial vegetation, habitat, and wildlife. (See
Appendix B, section V.)

Sand in beaches and other deposits along the channel
margins will adjust, probably within a few years, to
any pattern of fluctuating flow. During adjustment,
beach area is lost because of bank failure. The higher
the peak flow during fluctuation, the greater the loss
that occurs before the stable configuration is reached,
and the smaller the stable area remaining for camping
and vegetation. Loss will be greatest in narrow
reaches because those reaches experience a greater
change in water level for the same fluctuation range
than do wide reaches. Although floods may redeposit

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